Using novel methodology developed during the first phase of this project, highly purified sarcolemma will be obtained from rabbits and sheep at defined stages of pregnancy. The preparations will be characterized on the basis of receptor populations to agonists and antagonists of uterine contractile function and for their ability to specifically sequester or bind calcium ions. Since we can predictably alter uterine function by exposing rabbit uterine strips to steroid treatments in vitro, we expect to change sarcolemmal properties acutely using the same approach. Work with isolated sarcolemma is being related to the functional properties of intact cells in isolated uterine strips (excitability) or small disks (calcium movements). The research is aimed at understanding the basis for steroidal control over uterine function before and during normal and pre-term labor.